1. Field of the Invention
This invention pertains to the recovery of geothermal brine from liquid dominated subterranean zones and to the recovery of valuable metallic values present in the brine.
2. Description of the Prior Art
It has been recognized for many decades that the geothermal brines of the California Imperial Valley have the potential for recovery of valuable minerals such as copper, zinc, lead and silver in addition to such minerals of lower value such as potassium or lithium. However, with the advent of higher prices of energy in the 1970-80 period, attempts to recover these elements have been superseded by the incentives to produce electrical power from the thermal resources in the brine. Most of the development during this period has gone into solving the problem of handling the immense amount of silica and silicate scale which is produced as these brines are converted to steam. These silica and silicate scales, which are deposited along the pipes and fittings as well as in the flash vessels, consist of about 98% silica and iron silicates and 2% metallic sulfides. Some of the metallic sulfides tend to deposit near the well bore, but most of these sulfides intermix with silicates and form a scale mixture which is difficult to handle. It is very difficult to recover the diluted metallic values from such scale mixtures. Since some pure or undiluted sulfide scale contains about 10% by weight combined lead, copper and zinc and 0.3% by weight silver, it is highly desirable to collect the sulfide scale undiluted and substantially free of the silica and silicate scale.
Some samples of unflashed geothermal brine have shown a silver content from about 0.5 to about 2 parts per million (ppm). For such brines, a 55 megawatt (MW) plant would require about 120 million pounds of brine per day, and the equivalent silver production would be 120 pounds or 1440 troy ounces per day. If all of the silver were recovered from a geothermal brine which contains one ppm of silver, then at $10 per ounce, the revenue from the recovered silver would be 10.9 mills/kwh.
It is estimated that the plant in the above example would produce 50 to 100 tons of scale per day on a dry basis. Under present practice, only a small fraction of the silver is deposited in the scale and much is simply reinjected. That which comes down in the silica/silicate scale is very diluted and is extremely difficult to recover economically, not just because of being low in grade, but because the total tonnage of scale produced is quite low by normal production standards in the mineral industry. Thus, it can be appreciated that it would be very desirable to separate the valuable minerals in some form apart from the much larger volume of silica/silicate scale to obtain richer concentrates which could then be processed for ultimate conversion to pure metals.
As mentioned before, a part of the lead, copper, zinc and silver in the brine tends to separate as a sulfide scale. However, since there is insufficient natural sulfide in the brines to precipitate all of the heavy metals, some have suggested adding additional sulfide to the geothermal brine before flashing to produce additional sulfide scale.
U.S. Pat. No. 4,127,989 describes a process for maintaining the geothermal brine at a pressure above the bubble point, adding sodium sulfide to form additional sulfide precipitate and filtering out the heavy metal sulfides under pressure. Manganese, iron, copper, silver, lead and zinc are said to be recovered in this manner. The major disadvantages of this process are the codeposition of silica due to the alkalinity of sodium sulfide, the formation of hard-to-filter sulfide slimes, and the large volume of the surface tanks required to permit settling of the sulfide precipitates before filtering. The tanks in such a process have to be under a pressure of about 700 psia in order to maintain the brine above its bubble point. Such pressure duty would make the tanks and the process quite costly. These practical difficulties probably are the reasons why such a process has not been further developed or commercialized.
As an alternative, injection of sulfide to the bottom of the well would enable the wellbore to be used as the reaction vessel; and then, only the settling and filtering units would have to be supplied at the surface. However, this would still require quite a large pressurized holding volume to permit settling of the finely divided sulfides.
Another approach to recovery of heavy metals by sulfiding was reported in a publication entitled "Recovery of Heavy Metals from High Salinity Geothermal Brine" prepared for the U.S. Bureau of Mines by Standard Research International. This method involved treatment of spent amounts to recover zinc, lead, copper and silver. In order to recover all of the zinc, it was necessary to sulfidize some of the iron and manganese. Unfortunately, this makes the process uneconomical, because the iron and manganese have less economic value than the sulfiding agent. Furthermore, by treating only the spent brine, there is no possibility of recovery of the more valuable elements such as silver or possibly gold which will be lost in the silica/silicate scale during the subsequent flashing process.
U.S. Pat. No. 4,016,075 discloses a process for removing all of the metals including iron and manganese along with silica from geothermal brine by using ammonium hydroxide to raise the pH and form oxides and hydroxides. Ammonia is subsequently recovered by lime addition. Unfortunately, this process for treating geothermal brines generates huge volumes of sludge and make, the less abundant, but more valuable metals very difficult to recover economically. As in the other prior art methods, economic drawbacks are the reason why this process has not been commercialized.
Accordingly, there is a need for a simple and economical method for recovering the valuable metals such as gold, silver, copper, lead and tin from geothermal brines, and by so doing, offset the cost of producing electrical power therefrom. The present invention provides a long sought solution to this problem, and in so doing, decreases the corrosion rate of the metal casing, reduces the formation of scale in the well bore, and facilitates the conversion of hot-pressurized brine to steam.